Cycloaliphatic epoxy oligosiloxane resins with a high degree of condensation (>85%) were synthesized by a nonhydrolytic sol–gel reaction using 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane (ECTS), diphenylsilanediol (DPSD), and triphenylsilanol (TPS). Cycloaliphatic epoxy hybrimers with 2 mm thickness fabricated by thermal curing of cycloaliphatic epoxy oligosiloxane resins with a hardener and catalyst were optically transparent (∼90%) with a high refractive index of up to 1.583. The fabricated hybrimers also show high thermal resistance having no yellowing during thermal aging at 120°C for 1008 h and a high decomposition temperature (>300°C). On the strengths of these characteristics, the hybrimers are expected to find application as LED encapsulants. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011.